The present invention relates to a new device for measuring the stresses applied in use to the downhole assembly of a drill pipe.
French patent specification No. 2,041,342 which corresponds to U.S. Pat. No. 3,686,942 and U.S. Pat. No. 3,855,853 already describe devices of this type comprising measuring means formed by strain gauges secured to a stress-responsive resilient member, such as a metal sleeve of relatively small thickness which interconnects two conduits respectively coupled to first and second portions of the downhole assembly of a drill pipe.
A protective sleeve integral with one of the conduits protects the stress-responsive resilient member against radial pressure, and safety means limit the relative displacement of the conduits resulting from the stresses applied to the lower end of the drill column, thereby preventing deterioration or breaking of the resilient member as a result of a temporary rise in the applied loads.
These prior art devices are, however, subjected to large high longitudinally directed loads due to the hydrostatic pressure of the drill mud, and to a piston or jack effect resulting from a reduction in the cross-section of flow of the drilling fluid through the downhole assembly of the drill pipe (bottom effect).
For example if, in a wellbore having a 91/2 inches nominal diameter no special provision has been made for obviating the above-described drawbacks, a hydrostatic pressure of 600 bars can develop an axial compression load of 70 tons on the stress-responsive resilient member while the forces to be measured are comprised of between 0 and 40 tons. Similarly, the thrust resulting from the above-defined bottom effect may result in an axial traction load of 20 tons when the fluid pressure inside the device exceeds by 200 bars the external pressure, or alternatively, in an axial compression load of 20 tons if the external pressure exceeds by 200 bars the internal pressure.
The measurements performed by the stress-responsive resilient member must accordingly be corrected and the value of the applied correction may greatly exceed the range of the measured values. Such corrections can only be made when the pressures are known with sufficient accuracy, otherwise no measurement can be effected.